1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to an overcurrent sensing circuit that senses an overcurrent flowing in a power MOS field effect transistor.
2. Description of the Related Art
Semiconductor elements are vulnerable to irregular conditions, i.e., to overheating or overcurrents in excess of the rated current value, such conditions tending to bring about degradation of characteristics. Protection circuits for protecting semiconductor elements against irregular conditions have therefore been necessary to prevent degradation of characteristics as semiconductor elements, and a variety of overcurrent sensing circuits have been disclosed in the prior art for the purpose of protecting semiconductor elements.
FIG. 1 shows one example of such a circuit disclosed in U.S. Pat. No. 4,553,084. Power MOS transistor 30 is a transistor comprising 3,000 or more cell transistors, the gates, drains, and sources of each being connected in parallel, and has its source connected to ground 25 and its drain connected to power supply 29 by way of load 31. MOS transistors 24 and 27 are both of a single-cell structure and, similarly to the drain of MOS transistor 30, both have their drains connected to load 31. MOS transistors 24 and 27 also have their sources connected to ground 25, MOS transistor 24 by way of constant-current source 26 and MOS transistor 27 by way of sensing resistor 28. Comparator 33 has its first input terminal 34 connected to the source of MOS transistor 24 and its second input terminal 35 connected to the source of MOS transistor 27, and the output of comparator 33 is connected to output terminal 36. A gate drive (not shown) controls the gates of these MOS transistors 30, 24, and 27 in common by way of gate terminal 37. When the resistance of the cell-unit MOS transistors is 1500 ohms, the resistance of power MOS transistor 30 is approximately 0.5 ohms, and the resistance value of sensing resistor 28 is 30 ohms.
Here, when a current of 10 amperes flows from power supply 29 to power MOS transistor 30 by way of load 31, the current flowing through MOS transistor 27 is approximately 3 milliamperes, and accordingly, the voltage drop of sensing resistor 28 of approximately 100 millivolts is supplied to terminal 35 of comparator 33. Consequently, the current flowing in sensing resistor 28 is insignificant and its voltage drop is also small and has almost no effect on the load current. Moreover, the voltage drop of this sensing resistor 28 is proportional to the value of the current flowing, and this current value is proportional to the load current value flowing through power MOS transistor 30. Accordingly, the current flowing through sensing resistor 28 can be used for sensing the overcurrent of the load current flowing through power MOS transistor 30. Comparator 33 is used to detect whether or not the current is equal to the current flowing through constant-current source 26. When the current is equal to or below this value, the output of comparator 33 is 0. If the current of sensing resistor 28 exceeds the current of constant-current source 26, comparator 33 outputs a high-level signal warning of an overcurrent. The current of constant-current source 26 that serves as the alert standard can be set arbitrarily.
FIG. 2 shows an equivalent circuit diagram of the principal components of a semiconductor device disclosed in Japanese Utility Model No. 32543/92.
Power MOSFET 41 of this example of the prior art is composed of a large number of MOSFET elements arranged in parallel similar to power MOS transistor 30 of the first example of the prior art described hereinabove, and each of the sources, gates, and drains of the unit elements are coupled in parallel to form a source, gate, and drain as for a single element. This example is characterized by forming, within the same element as output power MOSFET 41, an overcurrent sensing circuit section that senses an overcurrent flowing through output power MOSFET 41 and load 47 by sensing the voltage drop across both ends of source wiring resistor 42 of this power MOSFET 41.
When a normal current flows across the source and drain of power MOSFET 41, the voltage generated at both ends of source wiring resistor 42 is equal to or less than a threshold value and the output of comparator 43 that inputs this voltage is low-level. Next, when an overcurrent flows across the source and drain, the above-described voltage exceeds the threshold value and the output becomes high-level. The absence or presence of an overcurrent is thus detected by sensing circuit 44 based on the output of comparator 43. In addition, ON/OFF control of power MOSFET 41 is effected from a sensing signal using logic circuit 45 and drive circuit 46.
In the case of the above-described U.S. Pat. No. 4,553,084, variance occurring in sensing resistor 28 exerts a direct effect on variance in overcurrent sensing. In other words, voltage drops due to sensing resistor 28 are compared at comparator 33 with a reference value set by constant-current source 26, but the occurrence of any variation in the resistance at the time of manufacture result in a proportional variation in voltage drop, and accordingly, variation in the overcurrent sensing value, and this results in the drawback that accurate overcurrent sensing is prevented.
Further, the use of source wiring resistor 42 in the sense resistance in Utility Model 32543/92 imposes a limit on the resistance that can be set, resulting in the drawback that variation in the resistance value exerts a direct influence on variation in the overcurrent sensing, as in U.S. Pat. No. 4,553,084.